The Air Accidents Investigation Branch said in an interim report Friday that the right engine on the Eurocopter EC135 Type 2 helicopter "flamed out and shortly after the left engine flamed out" as it flew over the Scottish city on Nov. 29.

It said further investigation would try to determine why both engines failed when the helicopter had 168 pounds (76 kilograms) of fuel in its tanks.

It also will seek to understand why the helicopter sent no emergency signal before it slammed into the Clutha Vaults pub with such force that the roof collapsed.